The invention is in the field of shelter structures and relates particularly to structures using lightweight roof membrane shapes. Examples are air-supported structures, tension structures, tent-like structures, arch-supported membranes, and free-span, point-supported systems. A survey of fabric tension structures for permanent buildings can be found in a presentation by the inventor herein to the International Symposium on Spatial Roof Structures at Dortmund, Germany on Sept. 10, 1984 entitled "A Decade of Fabric Tension Structures for Permanent Buildings." Additional background material on such structures can be found in the 12 references cited at pages 19 and 20 of the presentation. An example of such a structure is shown in U.S. Pat. No. 4,581,860 granted to the inventor herein. The presentation and its 12 references and said patent are hereby incorporated by reference in this specification as though fully set forth herein.
Tension structures depend upon shape and prestress for their stability and capacity of carrying loads. Tent-like tension structures are generally center-supported systems which may use suspension bridge concepts and double cantilevers. One or several peaks supported by central masts are the common forms. The "Florida Festival" structures at Sea World in Orlando, Fla. use the tent principles in a composition of vertical tents and their inversion. The roof membranes are supported by radial cables spanning from central poles to concrete edge beams. When internal supports are not desirable special means need to be developed to achieve free spans. Arch-supported structures are one method of developing free spans. One example is the Bullock's Department Store in San Jose, Ca. in which the roof fabric rides over a system of arches as in U.S. Pat. No. 3,807,421. Another method of creating free-span structures relies on point-supported systems; e.g., systems in which the free span is created by large A-frames supporting the main structural peaks from overhead in combination with peripheral poles restrained by stay cables. An example is the outdoor pavilion for the Crown Center in Kansas City, Mo. The integration of such A-frames into a balanced stress system is illustrated in the inventor's prior U.S. Pat. No. 3,773,061. Other examples of free-span tension structures are described or referred to in said presentation by the inventor herein and its references. For large and very large spans the use of a compression edge ring and short vertical compression struts together with a cablenet system is particularly effective. Said U.S. Pat. No. 4,581,860 granted to the inventor herein discloses a shelter structure using a saddle-shaped cable dome system for a large-span lightweight roof membrane. The structure uses the curvature of a saddle surface restrained with a warped edge ring, combined with two orthogonal cable nets separated by a set of compression struts to create an efficient structural system confined by the warped edge ring loaded primarily in compression.
The desirable characteristics of structures of this general type include high strength-to-weight ratio and ease and low cost of erection and maintenance. While much progress has been made in this field, it is believed that a need still remains to improve these and other desirable characteristics, and the invention is directed to meeting that need. The invention is particularly useful for facilities such as stadiums, arenas, and other large assembly spaces which use a horizontal edge ring and need high clearance in the center. Its objects include providing a structure which (i) can be erected simply and inexpensively, using commonly available materials and equipment, (ii) uses a structural fabric membrane forming a substantially continuous waterproof roof surface which avoids problems arising from temperature differentials and resulting geometric incompatibilities between roof and support structure, (iii) reduces and simplifies maintenance, and (iv) creates visual systems pleasing to the eye. The roof system can be translucent in whole or in part, and/or can be insulated. Provisions can be made for retractable roof covers.
In one exemplary embodiment of the invention the geometric configuration is such that compression forces act on a peripheral, edge support ring and on vertical or slanted compression struts which, together with a system of tension members (e.g. high strength cables) form a truss-like configuration. The cables are directly or indirectly supported by the edge ring, requiring no spinning, prefabrication, and multiple point lifting of ring cables. Main cables form simple spans on chords of concentric rings. The fabric membrane can form an integral part of the structural system. However, the stability of the main system is not dependent on the fabric. The system can be stressed by means of the vertical struts, avoiding the need for heavy stressing equipment and expensive adjustable cable connections.
A system which embodies an example of the invention uses a substantially rigid, generally laterally extending edge ring which has cable attachment nodes. Lower main outer cables connect respective edge ring nodes such that each of these cables intersects two other lower main outer cables at two respective lower outer strut nodes. An outer compression strut is supported at a respective lower outer strut node by two lower main outer cables and extends upwardly therefrom. The outer struts in plan view are spaced radially inwardly from the edge ring and form a concentric ring. Lower main inner cables connect respective outer struts such that each of these cables intersects two other lower main inner cables at respective lower inner strut nodes. Each inner compression strut is supported at a respective lower inner strut node by two lower main inner cables and extends upwardly therefrom. The inner struts in plan view are spaced radially inwardly from the outer struts and form a concentric ring. A particular dome structure can utilize a multiplicity of inner strut systems located on a set of concentric rings. An upper structure is supported by the edge ring and by upper portions of the outer and inner struts, and a roof membrane covers at least a part of the system.
The upper structure comprises upper main outer and inner cables. The upper main outer cables connect respective edge ring nodes and each intersects two other upper main outer cables at two respective upper outer strut nodes, to which are secured upper portions of respective outer struts. The upper main inner cables connect upper portions of respective outer struts and each intersects two other upper main inner cables respective upper inner strut nodes, to which are secured upper portions of respective inner struts. The upper and lower outer strut nodes are secured to the edge ring nodes by respective upper and lower outer radial cables, and the upper and lower inner strut nodes are secured to the upper outer strut nodes by respective upper and lower inner radial cables.
In one variation the structure adds, on top, substantially rigid arches extending radially inwardly from the edge ring nodes. Each arch is supported at the edge ring and at upper portions of a respective outer strut and respective inner struts. Radial cables need not be used, but may be added if desired, particularly if they can be used to simplify the erection process.
This exemplary embodiment of the invention allows for particularly efficient erection. For example, in the case of the variation which does not use arches, after construction of the edge ring and any supports which raise it above grade, the lower main outer cables are hung from the edge ring nodes, one at a time. Then they are moved and connected to each other as needed. Next, the upper main outer cables are hung from the ring to a position inverted from their final position, i.e. to a position similar to that of the lower main outer cables. Then the outer struts are connected to hang from the inverted upper main outer cables. The outer periphery of the system is then completed by moving the outer struts upwardly and connecting them to the lower main outer cables. Then the first ring is completed by adding the upper and lower radial cables. The erection process for the inner ring of the structure is similar, except that the main inner cables are hung from the upper outer strut nodes. If a system has more than one inner ring, the process is repeated for the additional inner rings. The system is stressed by jacking the struts to move the respective upper and lower strut nodes apart. Roof fabric is then attached. The center of the structure can be formed by a central compression strut secured by lower and upper central radial cables connecting its lower and upper portions, respectively, to the upper inner strut nodes.
The variation which uses arches can be erected similarly. The edge ring and the lower and upper outer main cables are erected as in the previous variation. These cables are then moved and connected to each other as needed to define the lower outer strut nodes. The outer struts are then installed as in the previous variation. The radial outer segments of the arches can then be placed in their approximate final positions by attaching them to the edge ring and to the respective upper outer strut nodes. The inner main cables and inner struts can then be erected similarly, except that they are supported by the upper outer strut nodes rather than directly by the edge ring, and the next inner segments of the arches can be placed in their approximate final positions, resting on the respective outer upper nodes and inner upper strut nodes. If one or more further inner rings are to be constructed, the same procedure is followed. A central strut supported at its bottom end by radial cables secured to the upper nodes of the innermost struts, can be used as in the previous variation. The centermost segments of the arches can be supported by the upper portion of the central strut and the innermost upper strut nodes. Final prestressing can be done by expanding the struts, e.g. by providing telescoping struts, and then the structure can be covered with roof skin, such as fabric. One configuration of the fabric roof structure consistent and effective with this cable dome system has radial segments spanning between radially oriented valley cables each connecting the tops of related struts and the related edge ring node. In the center of each segment the fabric is supported by a ridge cable supported by flying struts suspended from the upper nodes of the cable dome system. In case of the arch reinforced variation the arches can, but need not, replace the valley cable.